Project Summary Within the last decade, major progress has been made in understanding the genetic basis of Autism Spectrum Disorders (ASDs). Based on exome sequencing studies and microarray-based genotyping, it is recognized that the genetic architecture of ASD consists of rare mutations of large effect, including structural variants (SVs) and de novo point mutations that impact genes, as well as a significant contribution from common polygenic variation. However, a majority of the genetic risk for ASD remains unexplained. A proportion of the missing heritability of ASD could be attributable to genetic variation that remains inaccessible to today?s high throughput sequencing platforms including a majority of structural variants (SVs) and sequence variation that occurs within repetitive sequences in the genome. A systematic analysis of these novel classes of genetic variation could close a significant gap in our knowledge of ASD genetics. The development of new single- molecule sequencing platforms now enables direct sequencing of long DNA fragments (average read lengths >5,000 bp). These technologies have enable sequence assembly and variant calling within complex and repetitive regions of the genome and have dramatically increased the proportion of structural and tandem repeat (TR) variation that can be captured by whole genome sequencing (WGS). The application of long read WGS to ASD family samples could greatly expand knowledge of the genetic causes of autism. This study will investigate the contribution of complex genetic variants to risk for ASD using a combination of long-read and short-read technologies. (1) We will characterize global patterns of genetic variation in a primary sample of ASD families (N=373 cases, 127 sibling controls and their parents) by WGS using the Pacific Biosciences (SEQUEL) platform, and these data will be combined with an existing WGS dataset of Illumina short reads on the same samples. (2) We will investigate the genetic association of novel classes of SVs and TRs in genes and in regulatory elements that control gene expression, and novel findings will be replicated in Illumina WGS data on 2600 ASD families from the Simons Simplex Collection (SSC) (3) We will then investigate the influence of novel risk alleles on clinical phenotype in families and experimentally confirm the effects of mutations on gene function. Our proposed study will expand our knowledge of the genetic architecture of ASD and identify novel risk alleles and genetic mechanisms underlying disease risk.